completion: remove redundant __gitcomp_nl() options from _git_commit()

Those two options are specifying the default values that
__gitcomp_nl() would use anyway when invoked with no options at all.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1216,7 +1216,7 @@ _git_commit ()
 {
 	case "$prev" in
 	-c|-C)
-		__gitcomp_nl "$(__git_refs)" "" "${cur}"
+		__gitcomp_nl "$(__git_refs)"
 		return
 		;;
 	esac